1. Field of the Invention
This invention relates to a mirror for reflecting ultraviolet rays, and particularly to an improvement in the film construction of an ultraviolet reflecting mirror.
The ultraviolet reflecting mirror according to the present invention is used, for example, in an ultraviolet ray exposure apparatus in the manufacture of integrated circuits.
2. Description of the Prior Art
To obtain a high resolution and a high through-put in an ultraviolet ray exposure apparatus, it is advantageous to use a light of short wavelength. Therefore, today, the wavelength of the ultraviolet rays used is shifting from 230-250 nm (hereinafter referred to as deep U. V.) to the vacuum ultraviolet range of 170-190 nm.
In ultraviolet reflecting mirrors it is popular to use an aluminum (Al) film in order to obtain a high reflection factor in the deep U.V. and the vacuum ultraviolet range. However, the Al film will be remarkably reduced in reflection factor in no overcoat is present thereon. For example, if the Al film is left in the atmosphere for a month, the reflection factor thereof will be reduced to .about.70% for a wavelength 170 nm used.
Thus, the conventional reflecting mirrors have used an MgF.sub.2 film or an LiF film as the overcoat. The film thickness is optically .lambda./2 when the wavelength used is .lambda..
FIG. 1 of the accompanying drawings is a graph showing the reflection factor of only the Al film, and FIG. 2 of the accompanying drawings is a graph showing the reflection factor when an MgF.sub.2 film having a thickness of .about.550 .ANG. was provided on the Al film. The reflection factors of the S component and the P component when the wavelength used is 170-200 nm (vacuum ultraviolet range) and the angle of incidence onto the mirror is 45.degree. are Rs and Rp, respectively.
As is apparent from the comparison between FIG. 1 and FIG. 2, the MgF.sub.2 film does not contribute to the improvement of the reflection factor. However, there is a contribution of a little less than .about.1% as phase correction. Accordingly, the effect of the MgF.sub.2 film is the prevention of the deterioration of the reflection factor of the Al film as the overcoat, namely, the improvement in the durability of the Al film.
However, the conventional reflecting mirror having such an overcoat has suffered from a disadvantage that if it is left in the atmosphere for thirty days after the formation of the film, the reflection factor thereof is reduced by 10-20% for a wavelength 180 nm used because the thickness of the MgF.sub.2 film is as small as about .about.550 .ANG..
Also, in Applied Optics, vol. 15, No. 9, September 1976, there is a description regarding the reflecting film of a Fabry-Perot type filter used in the ultraviolet range of a wavelength of 200 nm or more. This reflecting film is comprised of alternate layers of Al film having a thickness of .about.200 .ANG. and MgF.sub.2 film and LaF.sub.3 film on the Al film. The thickness of each layer of the alternate layers is optically .lambda./4 if the wavelength used is .lambda..
However, this reflecting film has a disadvantage that the number of the alternate layers is great (ten to twenty layers) because the thickness of the Al film is small. Conversely, if an attempt is made to form the alternate layers by two layers, there will also arise a disadvantage in that, because the film thickness of the dielectric material is small, the Al film is deteriorated in the ultraviolet range (particularly the vacuum ultraviolet range) and a high reflection factor cannot be maintained.